The field of the disclosure relates generally to vibration and acoustic noise reduction, and, more particularly, to methods and apparatus for damping vibration of, and acoustic transmission through, an aircraft or vehicle structure.
Many structures are subjected to structure-borne vibrations and acoustic noise from various sources. For one example, aircraft and vehicle structures typically include engines that produce vibration and acoustic noise during operation. Such structures also typically are subjected to other vibratory and/or acoustic sources, such as those generated by aerodynamic forces. As a result, many such structures include systems intended to inhibit structural vibration and acoustic noise from reaching a passenger cabin. However, at least some such systems require separate devices to damp structure-borne vibration and to absorb acoustic noise.
For example, at least some known aircraft and vehicle structures are provided with constrained layer damping devices, in which one side of a layer of vibration damping material, such as a sheet of rubber or polyurethane, is coupled to the structural surface and an opposing side is coupled to a constraining layer. For a vibratory deformation at any location in the damping layer, the constraining layer induces shear deformation within the damping layer in directions parallel to the structural surface. The induction of shear deformation in the damping layer correspondingly dissipates a portion of the vibrational energy. However, such known constrained layer damping devices provide damping that varies significantly with the vibration frequency and the environmental temperature, and they provide little absorption of air-borne noise. Some known constrained layer damping devices use a viscoelastic foam damping layer to improve the frequency and temperature range for effective damping, but such devices still dissipate energy only to the extent that shear deformation is induced in the damping layer parallel to the structural surface.
Moreover, at least some known aircraft and vehicle structures are provided with thermal-acoustic blankets positioned between the structure and a panel of the passenger cabin. The blankets include a material, such as fiberglass or lightweight open-cell foam, that absorbs air-borne noise. However, such known blankets provide little damping of structure-borne vibrations.
Additionally, at least some known structures use a sound-absorbing foam with an embedded actuator, sometimes referred to as “smart foam.” Such known smart foam devices include a layer of light-weight foam with a flat base and an opposing arcuate upper surface. The flat base is coupled to the structure and the arcuate surface is coupled to a thin piezoelectric film. To supplement the acoustic noise absorption provided by the foam, the piezoelectric film is actively controlled to expand and contract the foam to produce acoustic waves that cancel acoustic noise. However, most, if not all, of the deformation in the foam, normal to the structural surface, is actively induced by the piezoelectric film to generate noise-cancelling acoustic waves. Moreover, the foam is selected for its ability to provide an elastic support foundation for the actively controlled vibration of the flimsy piezoelectric film and to absorb acoustic energy. Consequently, the induction of deformation in the foam does not dissipate substantial vibrational energy from structure-borne vibrations. In addition, the piezoelectric film and active control system introduce an additional cost, weight, and complexity to the noise reduction device, for example, from auxiliary control components.